Electrode integrity checking

ABSTRACT

Impedance between potential sensing electrodes of a high impedance meter is measured by applying a ramp waveform through a capacitor coupled to one of the electrodes to generate a substantially constant current, a measure of impedance being obtained by comparing this to the potential developed across the electrodes when a different current is flowing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the verification of electrode integrityin a measuring instrument, particularly an instrument having a highinput impedance, such as an electromagnetic flowmeter or a pH or redoxor other chemical (or bio-chemical) probe-type meter.

2. Description of the Related Art

It has been proposed to measure resistance between electrodes byinjecting a constant current from a constant current source. This methodworks, but has the drawback that a complex circuit is required to injectthe constant current.

EP 336 615 discloses some alternative methods in which a signal isinjected by means of a capacitor. A drawback of these methods is thatthere are constraints imposed on the timing of injection and sampling ofthe signal (specifically, it is essential for pulses to be injected atthe beginning of each half cycle), and it may be difficult to obtainaccurate results due to the somewhat irregular shape of the voltagewaveform produced by the injected signal.

SUMMARY OF THE INVENTION

The invention aims to alleviate the above drawbacks and provide anarrangement in which a reliable measurement of electrode resistance canbe obtained from a relatively simple circuit.

In a first aspect, the invention provides a method of obtaining an insitu measure of impedance (or conductivity) between potential sensingelectrodes of a meter having a high input impedance, the methodcomprising applying a substantially linear voltage ramp waveform to acapacitor coupled to one of the electrodes to generate a substantiallyconstant current, and deriving a measure of impedance (or conductivity)by comparing the potential developed across the electrodes while theconstant current is flowing to the potential developed across theelectrodes when no current, or a different current, is flowing.

Throughout this specification, reference is made to potential sensingelectrodes; it will be appreciated that potential must be measuredbetween two reference points. One of these reference points may be anearthing point or some contact with a solution rather than aconventional “electrode” of the meter in question (for example, a pH orreference electrode). In the specification and claims, the term“potential sensing electrode” is intended to encompass any point fromwhich a potential can be sensed; the invention extends to measurementsbetween a single electrode and a solution using a suitable referencepoint.

An advantage of using a capacitor to inject the current is that complexswitching arrangements are not required to isolate the electrodes fromthe impedance measuring circuitry when the potential across theelectrodes is to be measured; conventional resistance measuringcircuitry is liable to interfere with measurement of potential as theelectrodes can typically source only a small current. Another advantageof the method is that, because the current is injected for a discreteperiod of time, the measurement has an opportunity to stabilise,enabling a reliable reading to be obtained without complex circuitry orcorrection required; this can be contrasted with pulsed measurement ofimpedance.

Preferably, a plurality of measures of potential are obtained while thecurrent is injected. This enables measurement to be averaged over aperiod of time, which may enable noise to be cancelled or readings to beaveraged to provide greater accuracy, and can provide surprisinglyimproved accuracy as compared to pulsed single measurements.Surprisingly, if only two readings are taken during the duration ofcurrent injection, significantly greater accuracy and consistency ofresults may be obtained, as the measurement is less susceptible totransients.

Preferably a measure of potential is obtained after a predetermined(relatively short) delay after commencement of injection of current.This enables the apparatus to settle, and allows any (small) straycapacitances between the electrodes to be effectively charged.

It will be understood that by substantially linear is meant that, withinthe limits of experimental accuracy required, the current generated bythe ramp is within a desired tolerance range while the measurement ismade. High input impedance is meant an impedance sufficient to ensurethat the potential measured across the electrodes is not significantly(within the limits of experimental accuracy required) affected byconnection of the potential sensing circuitry; ideally the impedancewill be at least 1M ohm, and typically 10M ohms, 100M ohms or higher. Bysmall current is meant a current that is typically at most a few microamps,but may be many orders of magnitude lower (less than 1 micro amp,less than 100 nA, 10 nA or even less).

The method preferably further comprises obtaining a measure of thepotential across the electrodes to derive therefrom a measure of aphysical property related to the potential, said measure of potentialbeing obtained by potential measuring means having a high inputimpedance without disconnecting said capacitor from said electrode.

In one preferred application, the physical property is flow rate, themethod being employed in an electromagnetic flowmeter. In anotherpreferred application, the physical property is pH, the method beingemployed in a pH meter. In a similar manner to a pH meter, otherchemical (or bio-chemical) conditions may be sensed, for example in aredox potential meter. In both cases, the measure of impedance can beused to detect conditions such as fouled or faulty electrodes, brokenwiring, absence of fluid and the like.

The method preferably includes comparing the measure of impedance to atleast one threshold, and signalling at least one suspected faultcondition in dependence on the results of the comparison.

The ramp waveform may be generated by any of a number of conventionalramp voltage generators.

A preferred arrangement which has the benefit of being simple and costeffective to implement is to couple the input of the capacitor to thejunction between a series resistor-capacitor circuit, the potentialacross the combination being switched between two potentials, the timeconstant of the circuit being greater than the measurement period.

Alternatively, a more complex ramp synthesiser, for example based on adigital to analogue convertor or conventional linear ramp generator maybe used.

The invention extends to both method and apparatus aspects, and it willbe appreciated that preferred features of the method may be applied tothe apparatus, and vice versa.

In a first apparatus aspect, the invention provides sensing apparatusfor a meter arranged to derive a measure of a physical property from ameasure of potential across sensing electrodes, the sensing circuitcomprising a potential measuring circuit having a high input impedanceand inputs arranged for connection to the electrodes and means forobtaining a measure of the impedance of the electrodes comprising acapacitor coupled between one of said inputs and means for generating asubstantially linear ramp voltage so that the ramp voltage generates asubstantially constant current through the electrode impedance, theapparatus further including means for deriving a measure of impedancebased on the difference in potential across the electrodes when thesubstantially constant current is supplied and when no current, or adifferent value of current is supplied.

Preferably, the apparatus includes a capacitor coupled to each input; inthis way, the absolute potential of the electrodes relative to thesensing circuit can be left floating.

The apparatus preferably further includes means for comparing themeasure of impedance to at least one threshold and means for signallinga suspected fault condition based on the results of the comparison.

The sensing circuit may be employed in a pH meter including a pH sensingelectrode, th sensing apparatus and an output circuit arranged toprovide a calibrated measure of pH based on the measured electrodepotential. The output circuit may comprise additional circuitry suppliedwith the output of the potential measuring circuit, or may be integratedtherewith, the potential measuring circuit providing an appropriatelyscaled output signal.

The calibrated measure need not be individually calibrated for aparticular apparatus, but may be scaled appropriately based on a generalrelationship between measured potential and pH for meters of a similardesign.

As mentioned above, another preferred application of the invention is inan electromagnetic flowmeter, and in particular in a flowmeter havinglow power consumption, such as a battery-powered flowmeter.

In such an application, the apparatus preferably further includescontrol means arranged to control application of current to fieldgenerating coils of the flowmeter and to control application of the rampvoltage to the capacitor to enable measurements of both flow andelectrode impedance to be obtained.

Preferably, also, the apparatus is arranged to apply a magnetic field tothe fluid while said substantially constant electrode current isapplied, and to obtain samples of electrode potential in the presence ofthe magnetic field and of the substantially constant current, and in thepresence of the substantially constant current alone. This may enabledetermination of both flowrate and electrode impedance, withoutrequiring prolonged application of an electromagnetic field; in thisway, power consumption may be reduced, as application of the magneticfield typically requires significantly more power than application ofthe substantially constant electrode current.

The method may be adapted for use with a flowmeter.

Most preferably, the method comprises applying a pulsed magnetic fieldto the fluid during application of said substantially constant electrodecurrent and obtaining successive first, second and third values ofelectrode potential respectively before, during, and after applicationof the pulsed magnetic field, all during application of thesubstantially constant electrode current. In this way, variations in thesubstantially constant current can be compensated for by averaging. Inaddition, the inventors have found that, unexpectedly, better resultsmay be obtained if a relatively short magnetic pulse is applied than ifa short electrical pulse is applied. Furthermore, application of themagnetic field generally requires substantially more power thanapplication of the electrode current, so application of a short magneticfield and a longer electric field may reduce power consumption.

To improve results further, further values of the electrode potentialmay be obtained in the absence of said substantially constant electrodecurrent, preferably at least one further value in the absence of amagnetic field, and another further value in the presence of a magneticfield. Preferably, magnetic pulses of alternating plurality areemployed; this may reduce hysteresis effects. In addition(independently) electrode currents of alternating polarity may beemployed; this may reduce the effects of polarisation. In both cases,references to alternating polarity (particularly in the case of currentsof alternating polarity) may be extended to include groups of pulses ofalternating polarity. For example, a sequence of pulse elements (whichmay have varying magnitudes or signs) may be followed by a similarsequence in which the polarity of each or at least the majority of thepulse elements is reversed; this may still inhibit long-termpolarisation without every consecutive element alternating in polarity.

The invention may be employed to provide an EM flowmeter with an ‘emptypipe detector’, which is required to ensure the flowrate output iscontrolled, usually driven down scale, under this empty or partiallyfull pipe condition.

The invention may also be applied to flowmeters having a permanentmagnet to generate a magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings in which:

FIG. 1 is a schematic illustration of a conventional electro magneticflowmeter;

FIG. 2 is a block diagram showing a flowmeter according to a firstembodiment of the present invention;

FIG. 3 schematically illustrates the filtering used to extract the flowsignal and electrode resistance signals;

FIG. 4 comprises timing charts for explaining the operation of theembodiment shown in FIGS. 2 and 3;

FIG. 5 is a block diagram of a second, simplified embodiment;

FIG. 6 depicts a third embodiment incorporating a voltage monitor;

FIG. 7 is a graph indicating a relationship between electrode resistanceand a measured output signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first embodiment of the present invention is shown in FIG. 2. Thefluid flow is passed through a pipe 1, which contains an insulatinglining 20 if the pipe is itself conductive. The signal measuringelectrodes 5A, 5B are mounted on opposite sides of the pipe 1. A fluidearthing electrode 19 is present in this embodiment, but can be omitted;if only two electrodes are used, then the potential across theelectrodes can be measured when a current is injected if earthing of thefluid is not required, for example in some battery-powered applications.An alternating magnetic field B is generated by coils 2A, 2B, excited byan excitation or coil driver circuit 3. The coils 2A, 2B and itsassociated magnetic circuit are designed such that a magnetic field isgenerated perpendicular to the fluid flow and the electrode plane. Anelectrical excitation is generated by a voltage ramp generator 9A whichis applied to one of the electrode 5A via capacitor 10A. The capacitorvalue for 10A is selected to be of a suitably low value, e.g. 100 F, sothat the linear voltage ramp generator 9A causes a small, approximatelyconstant, current to flow through capacitor 10A and hence into theelectrode 5A and the fluid. Similarly a second ramp generator 9Bgenerates a constant current through capacitor 10B and electrode 5B.This constant current, which is proportional to the rate of change ofvoltage with time (dV/dt), flows through the equivalent resistance,which is the equivalent circuit of the electrode, surface coating, fluidconductivity and its wiring, represented by an equivalent resistance Recausing a voltage to be developed, hereinafter known as the ‘electroderesistance signal’. This voltage is measured and amplified, with theinduced EM flow signal, by the differential amplifier 6 fed fromelectrodes 5A, 5B. EM excitation timing is defined by timing circuit 4with the timing of the electrical ramp generator excitation is definedby timing circuit 11. The output voltage, consisting of the EM flowsignal and ‘electrode resistance signal’ is fed to two filters 12, 13.The input to these filters can be continuous time or sampled time, inthe latter case the sampling switch 17, controlled by a master clock 18provides this function. The output of filter 12 is proportional toflowrate, filter 13 is proportional to ‘electrode resistance signal’.The output of filter 12 is then further processed by conditioningcircuit 14 to generate the desired output signal such as 4-20 mA. Theoutput of validation filter 13, produces 2 output signals correspondingto the electrode resistance of electrode 5A to ground and electrode 5Bto ground.

These two signals can be used independently or jointly in a decisionblock/trip 15 to determine if the electrodes circuit is workingcorrectly, which includes electrodes 5A, 5B, coating on 5A, 5B, liningcoatings, wiring, fluid level, or damage. If a fault is developed suchthat the electromagnetic flowmeter will not operate correctly, then analarm signal is generated, which will drive the flow indication to asafe condition, usually downscale. The decision/trip circuit 14 can alsogenerate a warning signal to indicate a problem is present ordeveloping, such as electrode coating. The two electrode resistancesignals are also summed to generate a combined electrode resistancesignal, which could be output to indicate fluid conductivity.

The separation of the electromagnetic and electrode resistance signalsis further detailed below.

FIG. 4 is a diagram illustrating the timing associated with a lowpowered embodiment of this invention. The electromagnetic drive, usuallya constant current 34 is pulsed in one direction, off, then in theopposite direction, then off. This generates a corresponding magneticflux inside the meter, which generates a voltage of a similar waveformshape. An voltage ramp signal from either 9A or 9B generates acorresponding signal 35 or 36. The resulting constant currents into theelectrodes 32 or 33. The effective impedance of the electrode, itscircuitry and the fluid generate the corresponding electrode resistancesignal, this has similar waveform shape to 32 or 33, but is superimposedonto the electromagnetically induced flow signal similar in shape to 34.The sum is illustrated in the electrode differential signal 37.

In order to extract the separate flow and validation signals thecombined signals are processed by filters 12 & 13, which are illustratedin more detail in FIG. 3. Here the combined signal is sampled usingsample gate 17, signal 37, producing sampled signal 39. Use of a sampledwaveform simplifies processing and understanding although the principalbelow operates for a continuous analogue signal. To extract theelectromagnetic signal a series of Finite Impulse Response (FIR) combfilters are used. The first stage comb filter illustrated performs anA+C−2B function on the waveforms identified on trace 39, generating anoutput signal 40. It will be noticed that this processing has removedall trace of the electrode resistance signal. The reason for using suchprocessing is to rejects electrochemical and flow noise which tends tobe predominantly low frequency in nature, often with a 1/f frequencycharacteristic. The EM filter stage 2 performs a waveform H−G operation,effectively demodulating the drive signal and giving the desired flowsignal, which is not affected by the superimposed electrode resistancesignal.

To extract the electrode resistance signal the combined signal 39 is fedto Validation comb filter stage 1, which performs the functionillustrated by D−A and F−C, giving signal 42. These two signals may beused individually or combined in the illustrated Validation filter stage2 to give one signal which is (D−A)+(F−C).

For the comb filters in FIG. 3 to operate correctly the delay times T1,T2, T3 & T4 must be set to match the excitation waveform. By definingthe delays such that:

T 2, T 3=n×100 ms

where n is a positive integer

then this circuit also rejects all mains bourne interference, at both 50Hz or 60 Hz and all harmonics of these frequencies, on the recovery ofboth the EM signal and the electrode resistance signal. This techniqueis based on the invention patented by the author in UK Patent GB 2 271639. To match the electromagnetic excitation

T 1=Ton.

Similarly then it follows that to give the desired function

T 4=2×Ton

In the case of the above analysis it has only been assumed that oneelectrode circuit is driven at a time from either 9A or 9B. Clearlythrough symmetry it is possible on one electromagnetic drive cycle todrive say 9A, then on the next cycle drive 9B. That way its is possibleto essentially continuously validate each individual electrode.

An advantage of this invention is that the electrode resistancemeasurement is made using capacitive coupling with a low value capacitor10A, 10A, which has a benefit of not loading the electrode circuit withany resistive losses. The very high input resistance of differentialamplifier 6 is not degraded, ensuring that when electrodes do becomeslightly coated and resistive, or metering low conductivity fluids suchthat the flowrate reading is not shunted and accuracy affected.

In a further embodiment of this invention the voltage ramp generators 9Aand 9B are approximated by a simple resistor capacitor combination asillustrated in FIG. 5. With this arrangement an exponential ramp isgenerated by a voltage step from switches 22A, 22B fed to resistor 21 a,21 b and capacitors 20 a, 20 b. This exponential is sufficiently closeto linear ramp that the signal processing of FIG. 3 rejects the unwantedcross signal components. This arrangement has a further advantage thatit the capacitor combination of 10 and 20 act as a radio frequencyinterference filter, reducing susceptibility to unwanted radiofrequencies.

In a further embodiment of this invention, illustrated in FIG. 6, theelectrode voltages are buffered 40, 41 then fed to a voltage monitoringdetector circuit 42 which measures the voltage on each electrode. If anelectrode develops an open circuit or the pipe 1 is empty then theconstant current from capacitors 10A, 10B can attempt to generate a highsignal or voltage level which could overload later circuitry, such anAnalogue to Digital converter that would be required for subsequentprocessing in digital form. Such a detector 42 will enablesaturation/limiting of any processing to be detected.

The invention has been described above in the context of anelectromagnetic flowmeter. As mentioned above, the invention may beapplied to other meters such as pH meters. In such a case, simplercontrol circuitry is required, as it is not necessary to provide drivecurrent at appropriate times for field coils. In a pH meter, forexample, all that is required of the control circuitry is to apply theramp waveform and measure the difference between the potential acrossthe electrodes in the presence of this waveform and in its absence; thiswill give a measure of the electrode impedance. The pH can be derived inthe normal manner from the potential measured in the absence of the rampwaveform.

The above description concentrates on measuring impedance. It will beappreciated that conductivity, or any other related parameter can bederived in a similar manner; conductivity is merely the reciprocal ofimpedance. All references to obtaining a measure of impedance areintended to encompass any such related quantities.

What is claimed is:
 1. A method of obtaining an in situ measure ofimpedance between potential sensing electrodes of a meter having a highinput impedance, the method comprising applying a substantially linearvoltage ramp waveform to a capacitor coupled to one of the electrodes togenerate a substantially constant current, and deriving a measure ofimpedance by comparing a potential developed across the electrodes whilethe constant current is flowing to a potential developed across theelectrodes when no current, or a different current, is flowing.
 2. Amethod according to claim 1, wherein the ramp waveform is generated bycoupling an input of a capacitor to a junction between a seriesresistor-capacitor circuit, the potential across the combination beingswitched between two potentials, and the time constant of the circuitbeing greater than a measurement period for deriving said measure ofimpedance.
 3. A method according to claim 1, wherein the ramp waveformis generated by a voltage synthesiser.
 4. A method according to claim 1,wherein a plurality of measures of potential are obtained while saidconstant current is generated.
 5. A method according to claim 1, whereina measure of potential is obtained after a predetermined delay aftercommencement of generation of the constant current.
 6. A methodaccording to claim 1, wherein the physical property is pH, the methodbeing employed in a pH meter.
 7. A method according to claim 1,including performing a comparison by comparing the measure of impedanceto at least one threshold, and signaling at least one suspected faultcondition in dependence on a result of the comparison.
 8. A methodaccording to claim 1, further comprising obtaining a measure of thepotential across the electrodes to derive therefrom a measure of aphysical property related to the potential, said measure of potentialbeing obtained by using potential measuring means having a high inputimpedance and without disconnecting said capacitor from said electrode.9. A method according to claim 8, wherein the physical property is flowrate, the method being employed in an electromagnetic flowmeter. 10.Sensing apparatus for a meter arranged to derive a measure of a physicalproperty from a measure of potential across sensing electrodes, thesensing circuit comprising a potential measuring circuit having a highinput impedance and inputs arranged for connection to the electrodes andmeans for obtaining a measure of the impedance between the electrodescomprising a capacitor coupled between one of said inputs and means forgenerating a substantially linear ramp voltage so that the ramp voltagegenerates a substantially constant current through the electrodes, themeans for obtaining a measure of impedance being arranged to derive saidmeasure based on the difference in potential across the electrodes whenthe substantially constant current is supplied and when no current, or adifferent value of current is supplied.
 11. Apparatus according to claim10 including a capacitor coupled to each input.
 12. Apparatus accordingto claim 10 further including means for comparing the measure ofimpedance obtained by the means for obtaining the measure of impedanceto at least one threshold and means for signalling a suspected faultcondition based on the results of the comparison.
 13. A pH meterincluding a pH sensing electrode, sensing apparatus according to claim10, and an output circuit arranged to provide a calibrated measure of pHbased on the electrode potential measured by the sensing apparatus. 14.A pH meter according to claim 13 including control means arranged toapply said ramp voltage to obtain said measure of electrode impedanceand means for detecting a fault condition based on the measure ofimpedance, the meter being arranged to provide an output of a pHmeasurement or an indication that pH measurement may be invalid. 15.Sensing apparatus according to claim 10 for use in an electromagneticflowmeter, the flowmeter comprising field coils, the sensing apparatusincluding control means arranged to control application of current tothe field generating coils of the flowmeter and to control applicationof the ramp voltage to the capacitor so that measurements of both flowand electrode impedance can be obtained.
 16. Apparatus according toclaim 15, arranged to apply a magnetic field to a fluid while saidsubstantially constant current is applied, and to obtain samples ofelectrode potential in the presence of the magnetic field and of thesubstantially constant current, and in the presence of the substantiallyconstant current alone.
 17. An electromagnetic flowmeter comprisingfield generating coils, potential sensing electrodes and sensingapparatus according to claim
 15. 18. An electromagnetic flowmeteraccording to claim 17, further comprising output means arranged toprovide a measure of flow rate as said measure of the physical propertyor an indication that flow rate measurement may be invalid.
 19. Anelectromagnetic flowmeter according to claim 17 including means arrangedto provide an output signifying that a pipe through which flow is beingmeasured by said flowmeter may be empty or partially empty.
 20. Anelectromagnetic flowmeter according to claim 17, arranged to produce anoutput signifying zero flow on detection of an impedance signifying thata pipe through which flow is being measured by said flowmeter is empty.21. A method of obtaining measurements of flow of a fluid and electrodeimpedance in an electromagnetic flowmeter having field generating coilsand potential sensing electrodes, the method comprising applying asubstantially linear voltage ramp waveform to a capacitor coupled to oneof the flowmeter sensing electrodes to generate a substantially constantcurrent, deriving a measure of impedance by comparing the potentialdeveloped across the electrodes while said constant current is flowingto the potential developed across the electrodes when no current, or adifferent current, is flowing, and deriving a measure of flow from thepotential developed across the electrodes when no current is flowing, orbased on a plurality of values of potential measured at a plurality ofdifferent values of said substantially constant current.
 22. A methodaccording to claim 21, wherein obtaining measures of flow and impedancecomprises applying a pulsed magnetic field to the fluid duringapplication of said substantially constant current and obtainingsuccessive first, second and third values of electrode potentialrespectively before, during, and after application of the pulsedmagnetic field, all during application of the substantially constantcurrent.
 23. A method according to claim 22 wherein magnetic pulses ofalternating polarity are employed.
 24. A method according to claim 21wherein a plurality of values of said substantially constant current areemployed, including currents of alternating polarity.